1. Field of the Invention
This invention relates to a surveying instrument for measuring angles. More particularly, the invention relates to a zero setting device in a surveying instrument.
2. Background Art
In general, in a surveying instrument for measuring angles, a rotary stand with a collimator is rotatably supported on a base stand so that the angle between sightings of targets, as collimated through the collimator, is measured from the rotation angle of the rotary stand with respect to the base stand. That is, the angle is set to zero which is indicated by an angle displaying unit when the collimator is set in a predetermined direction (or a reference direction). Under this condition, the angle which is then indicated when a subsequent sighting of target is collimated through the collimator is read from the angle displaying unit. The angular indication is provided by electrically measuring the rotation angle of the rotary stand and is digitally displayed on the angle displaying unit.
Accordingly, it is essential for the surveying instrument described above to have a zero setting means which, when the collimator is set in a reference direction, zeroes the value displayed on the angle displaying unit. In a conventional surveying instrument, a zero setting switch is employed as the zero setting means. That is, the angle value displayed on the angle displaying unit is set to zero by manually operating the zero setting switch once. It is true that the zero setting switch is advantageous in that it can be readily operated. However, it is still disadvantageous in that sometimes the zero setting switch is carelessly operated by the operator, as a result of which the measurement must be carried out all over again.
In order to eliminate this difficulty, in the past, the zero setting switch has been installed at a position where it cannot be carelessly operated, such as in a recess, or the zero setting switch is provided with a cover so that it cannot be operated without opening the cover. However, such positioning makes the operation of the zero setting switch mechanically difficult and worsens the operability of the surveying instrument.